The copolymer of ethylene (E) and hexafluoropropylene (HFP) exhibits elastomeric properties and is excellent in heat resistance, oil resistance and chemical resistance (particularly alkali resistance), just as other fluorine resin and elastomers are. The copolymer also has electrically favorable properties such as high insulation, low dielectric loss tangent and dielectric constant and low frequency dependency. Particularly, the copolymer is expected to do well as oil seal material for an automobile as it is excellent in amine and oil resistance and when using in this way, small swelling to oil is desired. In order to obtain such advantageous properties, various E-HFP copolymers have been developed (JP-A-49-31785, JP-A-49-83738, JP-A-49-83789, JP-A-49-133442, JP-A-50-83440, JP-A-50-145457, JP-B-50-15304, U.S. Pat. No. 2,549,935, U.S. Pat. No. 4,039,595, Journal of the Chemical Society of Japan, 1980, (1), page 112 to 120, and J. Polym. Sci., IX, No. 6, page 481 to 492 (1952)).
For example, JP-A-49-31785 describes that E-HFP copolymer becomes elastomeric when the ethylene content of the E-HFP copolymer is at most 5 mole based on 1 mole of HFP. However, only a copolymer containing at least 78% by mole of ethylene (fluorine content is 45.7% by weight, HFP content is at most 22% by mole) is disclosed in the Examples.
JP-A-49-83738, JP-A-49-83789, JP-A-49-133442, JP-A-50-83440, JP-A-50-145457 and JP-B-50-15304 also describe that a copolymer containing 10 to 50% by mole of HFP is preferable. However, the HFP content of the E-HFP copolymer described specifically in these publications (see Example 2 of JP-A-49-83738) is at most 36.9% by mole (fluorine content 57.6% by weight).
U.S. Pat. No. 2,549,935 describes that a copolymer containing 5 to 50% by mole of HFP is worth noting but merely an E-HFP copolymer containing 37.5% by mole of HFP is obtained in the Example.
U.S. Pat. No. 4,039,595 teaches an E-HFP copolymer which is an oily matter having a high viscosity with low or medium molecular weight containing 15 to 50% by mole of HFP. However, only an E-HFP copolymer which is an oily matter containing 40% by mole of HFP (fluorine content is 59.4% by weight) is obtained in the Example.
Journal of the Chemical Society of Japan, 1980, (1), page 112 to 120 reports preparing an E-HFP copolymer variously changing the charging amount of HFP and polymerization conditions and concludes that a E-HFP copolymer containing at least 50% by mole of HFP cannot be obtained. Thus merely a copolymer containing 39.8% by mole of HFP (fluorine content is 59.3% by weight, Experiment EF-12) was obtained at highest according to the Experiment Results (Table 2 and FIG. 1).
In this Experiment EF-12, the intrinsic viscosity (molecular weight) is presumed to be low and the Mooney viscosity ML (1+4, 100° C.) is presumed to be at most 50. As described in the reference, when the HFP content increases or the Mooney viscosity decreases, the tensile strength of the vulcanizate tends to decrease. From all of the above, in Experiment EF-12, vulcanization properties such as tensile strength when peroxide vulcanization is conducted are presumed to be insufficient for practice.
WO 94/24175 pamphlet describes an ethylene-HFP-TFE copolymer. However, the copolymer with the highest fluorine content described in the Examples has a fluorine content of 57.5% by weight (Example 4) and the properties of the vulcanizate (for example chemical resistance) are insufficient.
J. Polym. Sci., IX, No. 6, page 481 to 492 (1952) discloses an experiment of conducting emulsion polymerization by charging HFP and ethylene at a weight ratio of 50/18. However, the obtained copolymer was reported to have an HFP content of 7.6% by mole and a fluorine content of 23.1% by weight and to be an elastic body with low molecular weight and inferior mechanical strength.
In this way, attempts to prepare an E-HFP copolymer with a high HFP content have been made in the past. However, in reality, a copolymer comprising ethylene and HFP and also having a fluorine content of at least 60% by weight did not exist.
The present invention aims to provide an ethylene-hexafluoropropylene (E-HFP) copolymer comprising ethylene and hexafluoropropylene and containing at least 60% by weight of fluorine.
In addition, the present invention aims to provide a vulcanizable composition obtained by using the E-HFP copolymer and an elastomeric vulcanizate obtained from the vulcanizable composition, which has a small swelling ratio and thus particularly useful as sealing or hose material for an automobile.